GAZIANTEP, Turkey — A U.S.-backed effort to arm the moderate Syrian opposition is finally ramping up along the Turkey-Syria border, but it may come too late to save the rebels from defeats on two fronts, by President Bashar al-Assad’s government and by the extremists seeking to carve out an Islamic state.

Spurred by concerns that the al-Qaeda-inspired radicals will continue their relentless march across Iraq and Syria, the United States and its allies have begun accelerating the supply of arms and ammunition to a small number of vetted rebel groups in northern Syria, according to diplomats and rebels who have been receiving the deliveries.

Yet even as the fresh support arrives, challenges are mounting for the embattled moderates, who have been pushed out of eastern Syria by extremists, are being encircled in Aleppo by the government and are seeing their ranks eroded by defeats, desertions and infighting.

The outlook for the revolt against Assad’s rule is now bleaker than at any time in the past three years, rebel commanders say, diminishing the chances that the opposition will be able to present any meaningful challenge to the regime or even to serve as a counterweight to Islamist radicals, as U.S. policymakers are hoping.

“This is a very critical moment for the Free Syrian Army,” said Hussam al-Marie, a spokesman for the rebels in the north. “We are fighting on two fronts, and the situation is growing more urgent every day.”

A Syrian man walks along a street covered with dust after a reported airstrike by government forces in Aleppo on July 21. (Ahmed Deeb/AFP/Getty Images)

Covert support effort

The latest effort to supply the rebels is part of a covert support program, authorized by President Obama last year, under which the United States participates alongside Persian Gulf Arab and European allies in sending deliveries of arms, ammunition and money across the Turkish and Jordanian borders. A separate, openly acknowledged program to provide nonlethal aid such as vehicles, food and medicine has also gathered pace in recent months, U.S. officials say.

The covert program, which got off to a sputtering start because of concerns about the rising influence of Islamists, is separate from a far bigger $500 million proposal to train and equip the rebels requested by Obama last month. That package, framed as part of a broader counterterrorism strategy, marks the first major offer of American support to the rebels.

But the proposal is subject to congressional approval and, if implemented, would be unlikely to get underway at least until the middle of 2015.

By then, there may be few if any moderate rebels left to aid.

In recent weeks, the Islamic State fighters have built on their stunning gains in northern Iraq to snatch control of the eastern Syrian province of Deir al-Zour, further consolidating their control over the nation-sized swath of territory spanning Iraq and Syria that forms the nucleus of their self-proclaimed caliphate.

More seriously for the mainstream rebellion, gains by government forces in and around the northern city of Aleppo have steadily eroded the opposition’s grip over Syria’s largest metropolis. Now the rebels fear they could soon lose their foothold there altogether.

More than six months of punishing daily airstrikes, which have killed thousands and forced hundreds of thousands to flee, have all but cleared the city’s rebel-held portion of residents. Disillusioned fighters, starved of arms, ammunition and pay, have been drifting away from the front lines to find jobs to feed their families or, in some instances, to join the better-funded extremists.

A Syrian man carries a girl after a reported airstrike by government forces in Aleppo on July 15. (Baraa al-Halabi/AFP/Getty Images)

The rebels’ sweep into Aleppo in July 2012 was a defining moment for the revolt against Assad’s rule, putting them in control of more than half of the country’s commercial capital and offering the first demonstration of their ability to seize strategically significant territory.

Its loss could be a crippling blow, leaving the opposition with no significant hold in any important city and undermining its claim to hegemony in the north by shrinking its influence to the rural province of Idlib.

“If we lose Aleppo, we lose the revolution,” said Mohammed Abu Bahri, 23, a despondent former Syrian fighter who runs a cafe in the Turkish border town of Kilis. He left the front lines earlier this year after suffering his fourth injury in 18 months. He was hurt three times fighting government forces and the last time in battles with the Islamic State.

That wound has not healed, and he lacks money for medical treatment. Next month, he plans to join an accelerating stream of former fighters who are paying smugglers to ferry them to Europe.

“I never imagined it would end like this,” he said in the cafe, where fighters on break from the front lines frequently gather. Increasingly, the fighters are not going back.

“They have to find work to feed their families,” Abu Bahri said. “I could give you a list of more than 1,000 names of fighters who are sitting in Turkey waiting for support and weapons to go back to Syria.”

Other fighters have been defecting to join the Islamic State, which offers benefits on a scale unmatched by the moderate opposition, said Haitham Afisi, the deputy commander of the Supreme Military Council, a body representing moderate rebels.

“If we have support, there are lots of revolutionaries that would come back,” he said. “For now, the Islamic State is the only group that has everything — money, weapons and oil.”

Painstakingly vetted groups

U.S. officials and diplomats say there will be more assistance, but whether it will be enough to reverse the tide is in question.

The latest deliveries of arms and money are in part the result of improved coordination between the chief partners in the 11-member Friends of Syria alliance, according to U.S. officials and rebels. Saudi Arabia and Qatar, the opposition’s biggest backers, have halted their unilateral funding of rebel groups, which was blamed for the unchecked rise of Islamist factions. The countries are now cooperating with the United States and its European allies to ensure that supplies reach only U.S.-approved moderates.

The effort got a big boost in March, when the United States authorized the first deliveries to the rebels of U.S.-made TOW antitank guided missiles, the most advanced weapons yet to be delivered to the battlefield. The missiles have propelled some important gains against government positions in the province of Idlib, and they have also helped the rebels slow the pace of gains by Assad loyalists around Aleppo.

The supplies are, however, being funneled to just eight relatively small groups that have been painstakingly vetted for their adherence to moderate goals. The increase in their support has not been enough to offset the reduction in assistance to some of the bigger groups behind the revolt’s early gains, who are not extremists but are Islamists, according to Noah Bonsey of the International Crisis Group.

The squeeze has been most noticeable in Aleppo. There, the Tawheed Brigade, which led the initial sweep into the city and often received funding from Qatar, has seen its resources shrink and its fighters split into feuding camps.

Rifts between factions

The new supplies, moreover, are engendering rivalries between groups that had hitherto cooperated in uneasy but mostly effective alliances. Harakat Hazm, the group that is the biggest recipient of U.S.-authorized aid, has clashed in the past month with the Islamist group Ahrar al-Sham over control of the Turkish border crossing of Bab al-Hawa in Idlib. Last week, Harakat Hazm lost three senior commanders in a car bombing at one of its headquarters in Aleppo, an attack that was blamed on the government but that could have been carried out by rivals.

Over the past week, a new rift with the al-Qaeda-affiliated Jabhat al-Nusra threatened to open up a third front that would embroil the moderate rebels in yet more battles in their shrinking territory.

“This is as tough a set of conditions as the rebels have ever faced,” Bonsey said. “The new support does not seem quick or extensive or deep enough to enable the mainstream rebels to deal with all these emerging challenges.”

Liz Sly is The Washington Post’s Beirut bureau chief, covering Lebanon, Syria and the wider region. She has spent more than 17 years covering the Middle East, including the first and second Iraq wars. Other postings include Washington, Africa, China, Afghanistan and Italy.